Agar powder is basically processed and dried seaweed , or in higher grade, purified proteins forms. Usually this can be prepared by dissolving in water in various percentages, heating then letting it cool. You can purchase this from health food stores, or order them from chemical suppliers (but probably an overkill for a fruit yoghurt). There are different recipes you can try, and you can probably get ideas from health food stores, but I don't think it will thicken fruit yoghurt in the way you want it, because agar tends to be firm like jelly rather than thick depending on the percentage dissolved in water (unless it is hot).